Re: [PATCH for-next 3/4] io_uring: grow a field in struct io_uring_cmd

[Sagi Grimberg <sagi@grimberg.me>]

> Use the leftover space to carve 'next' field that enables linking of
> io_uring_cmd structs. Also introduce a list head and few helpers.
> 
> This is in preparation to support nvme-mulitpath, allowing multiple
> uring passthrough commands to be queued.
> 
> Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
> Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
> ---
>   include/linux/io_uring.h | 38 ++++++++++++++++++++++++++++++++++++--
>   1 file changed, 36 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/io_uring.h b/include/linux/io_uring.h
> index 54063d67506b..d734599cbcd7 100644
> --- a/include/linux/io_uring.h
> +++ b/include/linux/io_uring.h
> @@ -22,9 +22,14 @@ struct io_uring_cmd {
>   	const void	*cmd;
>   	/* callback to defer completions to task context */
>   	void (*task_work_cb)(struct io_uring_cmd *cmd);
> +	struct io_uring_cmd	*next;
>   	u32		cmd_op;
> -	u32		pad;
> -	u8		pdu[32]; /* available inline for free use */
> +	u8		pdu[28]; /* available inline for free use */
> +};

I think io_uring_cmd will at some point become two cachelines and may
not be worth the effort to limit a pdu to 28 bytes...